1. Field of the Invention
The present invention concerns a method and a control device to operate a magnetic resonance system to acquire magnetic resonance image data of an examination subject, in particular to acquire data representing the heart or a portion of the heart within the scope of cardio-MRT acquisitions.
2. Description of the Prior Art
Magnetic resonance tomography is a modality in widespread use to acquire images of the inside of a body. In this method the body to be examined is exposed to a relatively high basic magnetic field, for example of 1.5 Tesla or even of 3 Tesla in newer systems (known as high magnetic field systems). A radio-frequency excitation signal (what is known as the B1 field) is then emitted with a suitable antenna device, which causes the nuclear spins of specific atoms to be excited to resonance by this radio-frequency field and tilted by a specific flip angle relative to the magnetic field lines of the basic magnetic field. The radio-frequency signal radiated upon relaxation of the nuclear spins—known as the magnetic resonance signal—is then detected with suitable antenna devices (that can be identical to the transmission antenna devices). Finally, the raw data that are acquired in this manner are used to reconstruct the desired image data. For spatial coding, respective defined magnetic field gradients are superimposed on the basic magnetic field during the transmission and readout (recognition) of the radio-frequency signals.
Such magnetic resonance systems include a number of subsystems that must be activated within a predetermined measurement sequence under consideration of fixed temporal relationships within the scope of a measurement procedure. Among these subsystems area magnetic field system that includes, for example, a basic magnetic field system; a gradient coil system; and possibly a magnetic field shim system. Another subsystem is a radio-frequency system that includes the antenna arrangements and suitable transmission and/or reception systems in order to emit matching radio-frequency pulses via the antenna arrangements, and to process magnetic resonance signals detected by the antenna arrangements.
The magnetic resonance system also includes a control device for coordinated activation of the subsystems. With the use of the control device, prior to a diagnostic measurement, a number of adjustment measurements are implemented to adjust at least one of the subsystems in which an adjustment volume associated with the appertaining diagnostic measurement is taken into account, this adjustment volume encompasses at least one region of a body containing the subject of the examination. In these adjustment measurements the individual subsystems are calibrated to the specific properties of the examined subject. The adjustment measurements for the most part ensue as a “black box” procedure, meaning that the operator does not know in detail how the adjustments proceed, and instead the entire system or the controller is fashioned so that it implements the matching adjustments fully automatically for a specified diagnostic measurement, and thereby optimizes the necessary parameters.
Such adjustment measurements are normally not locally applied, meaning that the signal of the entire measurement volume of interest is considered in order to optimize the emitted radio-frequency, for example to implement a transmitter adjustment or to optimize the magnetic field shim. In measurements in which a very exact depiction of a specific examined structure (for example a specific organ such as the heart) is required, such an approach of non-localized adjustment has proven to be unsuitable. For example, in a non-localized measurement volume for the frequency adjustment, most often a frequency is determined that is not optimal for the imaging at the organ to be examined since most signal contributions originate from other tissue regions with a correspondingly deviating optimal frequency.
In an optimization method to optimize the frequency, multiple additional measurements are therefore implemented in order to achieve a manual optimization of the offset frequency in an image-based manner, for example. This means that images with different offset frequencies are generated, from which the one with the best image quality is sought by the operator in order to thus find the optimal frequency. Such additional measurements, however, cause additional stress for the patient.
In practice it is also possible by default in many pre-existing magnetic resonance systems for the operator to define what is known as a “bounding box”. A cubical or cuboid volume in which the organ to be examined should lie is thereby defined with the use of a graphical user interface in overview images of the patient. The definition of the “bounding box” is, however, relatively complicated and requires a number of optimization steps in order to optimally position the adjustment volume three-dimensionally. Usually a successive manual adaptation of the “bounding box” ensues by positioning and alignment on multiple localizer images with different slice orientations. This requires an additional time expenditure, during which the patient must remain longer in the patient tunnel of the apparatus, which is most often perceived to be uncomfortable. Such a complicated manual setup of the “bounding box” nevertheless can still result in a suboptimal definition of the adjustment volume, which also again directly affects the quality of the images.